1. Field of the Invention
The present invention relates to developer materials for electrostatic images, particularly for use in an electrostatic high speed printer and the like.
2. Description of the Prior Art
The developer material for use in electrostatic development comprises a mixture of smaller resinous toner particles and larger carrier beads made of iron or glass. The toner particles are held on the surface of the carrier beads by electrostatic force, which develops from the contact therebetween producing triboelectric charging of the toner particles and the carrier in opposite polarities. When the developer material is moved into contact with the latent electrostatic images formed on a photoreceptor, the toner particles are attracted to the latent images, and, thus, the images are developed. The developed images are transferred onto plain paper and fixed thereon by heating.
Generally, such a dry developer material has problems which are difficult to solve, such as toner filming of the carrier beads and throwing-off of the toner particles during agitation of the developer material in a developing station.
Here, the term "throwing-off" means that the toner particles are thrown out from the developing station and scattered during the rotation of the magnetic roll of the developing station by moving air, centrifugal force or mechanical impulse.
The toner filming occurs due to the recycling of the carrier through many cycles producing many collisions. The attendant mechanical friction causes the toner material to partly melt and stick on the surface of the carrier beads, thus, forming a film of toner thereon. The toner filming impairs the normal triboelectric charging of the toner particles in the developer mixture, because the normal toner-carrier triboelectric charging is partly replaced by a toner-toner relationship. The improperly charged toner particles can be deposited on the non-image areas, whereby the quality of the printed matter is impaired, because the non-image areas possess an unacceptable level of background.
When the toner filming grows to a certain degree, the entire developer material must be replaced, thereby, increasing the cost of operation of a development apparatus. Furthermore, the replacement is time consuming. This is especially significant in a high speed printer.
The throwing-off problem occurs due to weak electrostatic charging between the toner particles and the carrier beads. When the developer material is vigorously agitated in the developing station, the toner particles are attracted to and adhered on the carrier beads. However, some of the toner particles are apt to separate from the carrier and fly up to deposit on a non-image area of the photoreceptor, and also, on the inner surface of the printer. As a result, fogging of the images occurs and the performance of the optical system including the corotron is impaired. Consequently, the electrostatic charging must be large enough to avoid the throwing-off of the toner particles.
It is well known that the carrier beads can be coated with a resin so as to reduce the formation of toner filming. However, the resin must be easily and uniformly coated on the surface of core beads and firmly fixed thereon.
The coated carrier beads and the toner particles are required to exhibit at least the following features during operation.
(A) The carrier beads must effect the necessary triboelectric charging of toner particles and a sufficient number of toner particles must be held on the coated surface of the carrier beads to exhibit a high optical density of the images, and also, to reduce the throwing-off of the toner articles so as not to cause fogging of the images.
(B) The coating resin must not peel off from the core beads and must be resistant to abrasion. Even if the coating chips, flakes or spalls, the fine powder must not cause fogging of the images and corrosion of the developing station, and toxic materials must not be separated.
(C) The resinous material of the toner particles must not stick to the coating resin during agitation, that is, must reduce the formation of undesirable toner filming which impairs the normal triboelectric charging, and must not run on the paper during fusation, that is, must improve the resolution.
In the prior art, the developer material for use in an electrostatic high speed printer does not satisfy all of the features (A) through (C) above, although several patents disclose methods of improving the behavior of either the toner or the carrier.
For example, carrier beads are coated with a fluorine containing polymer and a modifying material which are physically mixed with each other. It is necessary in order to obtain a uniform coating that a diluted formulation of the polymer which includes chromium oxide be sprayed onto the core beads suspended in a cyclic fluidized bed tower. The process is complicated.